Kowalski et al., in U.S. Pat. No. 4,427,836 ('836), discloses the production and use of an aqueous dispersion of water-insoluble particulate heteropolymers made by sequential emulsion polymerization. The particles disclosed in the '836 patent have a core of polymeric acid encased in a shell polymer permeable to volatile bases, such as ammonia or an organic amine. The shell is not permeable to permanent, non-volatile bases such as sodium hydroxide. When the particle is exposed to a volatile base, the base permeates the shell and at least partially neutralizes the polymeric acid core, causing the core to swell because of hydration. Upon drying the aqueous dispersion and volatilization of the volatile base and water, an enlarged swollen particle containing a microvoid is formed. Because of the formation of microvoids during drying, the aqueous dispersions of the '836 patent is useful as an opacifying agent in water-based coating compositions.
Kowalski et al., in related U.S. Pat. No. 4,469,825 ('825) discloses core and shell polymer particles wherein the core is composed of polymeric base and is encased in a shell polymer permeable to an acid, such as acetic acid. The polymeric base core is neutralized and swollen with the acid. Similar to the '836 patent, an aqueous dispersion of the acid-swollen, polymeric-base containing core and shell particles of the '825 patent is useful in making water-based coating compositions, wherein microvoids are formed in the cores of the swollen particles upon drying.
Blankenship et al., in U.S. Pat. No. 4,594,363 ('363) also discloses a process for making core and shell polymer particles useful for opacifying coating films. The particles disclosed in the '363 patent are prepared by emulsion polymerizing a core from a core monomer system of at least one ethylenically unsaturated monomer containing acid functionality, encapsulating the core with a hard shell by emulsion polymerizing a shell monomer system in the presence of the core. The shell system is chosen such that the shell permits the penetration of fixed or permanent bases, causing swelling of the core at elevated temperature. When a dispersion of the swollen particles is dried, the microvoids cause opacity.
Kowalski et al., in U.S. Pat. No. 4,468,498 ('498) discloses a similar process as disclosed in '836 for making an aqueous dispersion of core and shell polymers. The particles disclosed in the '498 patent have a core containing sufficient acid groups to render the core swellable by neutralization with a volatile base and have a shell permeable to volatile base. These heteropolymer dispersions are useful in aqueous coating compositions.
Chip et al., in U.S. Patent No. 4,920,160 ('160) discloses core and shell polymeric particles having a core formed from monomers having acid functionality and a shell free of acid groups. The shell is permeable to non-volatile base, such as sodium hydroxide. The non-volatile base penetrates to the core, neutralizes it, causing swelling of the core. Swelling is achieved at elevated temperatures in the absence of organic solvent. These swollen particles, upon drying in a paint or other composition, have air in the voids and act as opacifying agents.
U.S. Patent No. 4,677,003 to Redlich et al. relates to a sequential polymerization process for preparing a water-insoluble dispersion of core and shell particles having a core containing a solvent blend. The core emulsion is prepared by emulsifying in water at high shear a mixture of; hydrophilic solvent; hydrophobic solvent; monomer, including carboxylic acid monomer; anionic surfactant; water-insoluble emulsion stabilizer; and water-insoluble thermal polymerization initiator. The core emulsion is heated to polymerize the initial monomer, forming the core particles. The mixture is a non-solvent for the polymer prepared. Subsequently, a base selected from ammonia and organic amines is added, neutralizing the polymerized carboxylic acid and forming core and shell particles. Optionally, additional monomer is added and polymerized on the core and shell particles. These particles are useful as opacifying agents in coating compositions and also for microencapsulation of organic target materials.
All of the patents discussed hereinabove lead to core and shell particles with microvoids in the center. However, none of the patents disclose core and shell particles with microvoids having a channel from the core, through the shell, to the exterior of the particle. It is an essential feature of all the particles disclosed in the above patents to have a core completely surrounded by the shell. For example, as stated in column 9, line 46-49 of U.S. Patent No. 4,468,498, properly encapsulated particles have alkali swellable cores that do not titrate with base when exposed to alkali metal bases. This implies the core is completely surrounded.
In general, channels from the core through the shell are undesirable if the particles are to be used as opacifying agents in coating compositions because the channels allow the coating binder to penetrate to the core, filling the microvoids and destroying their opacifying power.